Is there a general formula...?
Of course! ... Of(x) / course2=NOT(n)!
... you are best advised doing a few clip tests...
This!
Photography is difficult enough already
You seem to have (at least) one poorly developed formula, IMHO of course
Why do people shoot expired film (other than film which has been refrigerated)
"Better a fallen rocket than never a burst of light"
Time is your most valuable commodity, and film is cheap. Why risk that once in a lifetime shot (or even that shot of your dear Aunt Betty) with expired film? I don't understand the mindset.
Have you never lost a shot with fresh film? Maybe we should not blame the film. On the other hand, generally speaking, expired film is cheaper.
Best